Conventionally, there has been proposed a spark plug in which, in order to improve ignition performance and durability of its ground electrode, a noble metal tip is embedded into the ground electrode by means of resistance welding such that the noble metal tip projects from the distal end of the base member of the ground electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-307857 and Japanese Patent Application Laid-Open (kokai) No. 2007-87969. In the case of a ground electrode in which a noble metal tip is embedded into the ground electrode base member, in some cases, oxide scale is formed at a joint portion between the ground electrode base member and the noble metal tip due to heat generated in an internal combustion engine. Excessive formation of such oxide scale may result in separation of the noble metal tip from the ground electrode base member.
Heretofore, sufficient studies have not been conducted on the joint strength between a noble metal tip and a ground electrode base member in a spark plug in which the noble metal tip is joined to the ground electrode base member through resistance welding.
In view of the above-described circumstances, an object of the present invention is to provide a technique which can improve the durability of a spark plug.